Memory devices are an extremely important component in electronic systems. The three most important commercial high-density memory technologies are SRAM, DRAM, and FLASH. Each of these memory devices uses an electronic charge to store information and each has its own advantages. For example, SRAM has fast read and write speeds, but it is volatile and requires a large cell area. DRAM has a high memory density, but it is also volatile and requires a refresh of the storage capacitor every few milliseconds. This refresh requirement increases the complexity of the control electronics.
FLASH is the major nonvolatile memory device in use today. Typical FLASH memory devices use charges trapped in a floating oxide layer to store information. Drawbacks to FLASH include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of about 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the thickness of the gate oxide has to stay above the threshold that allows electron tunneling. This thickness requirement severely restricts the scaling trends of FLASH memory.
To overcome these shortcomings, magnetic memory devices are increasingly being evaluated. One such device is magnetoresistive random access memory (hereinafter referred to as “MRAM”). MRAM has the potential to have a speed performance similar to DRAM. To be commercially viable, however, MRAM needs to have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds. For an MRAM device, the stability of the memory state, the repeatability of the read/write cycles, and the power consumption are some of the more important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of a magnetic moment vector. The magnetic moment is a physical property of ferromagnetic materials.
Current MRAM devices typically comprise a magnetoresistive tunneling junction (MTJ) memory cell that comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers.
In typical MRAM devices, storing data is accomplished by applying magnetic fields and causing a magnetic material in an MRAM cell to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive state of the cell which depends on the magnetic state. The magnetic fields are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
The word line and digit line include conductive material through which a current can be passed. In one such system, described in U.S. Pat. No. 6,654,278 to Engel et al., the subject matter of which is herein incorporated by reference in its entirety, the word line is positioned on the top of the MRAM device and digit line is positioned on the bottom of the MRAM device and is directed at a 90° angle to word line. It is not necessary that conductors such as word line and digit line be in physical contact with the other layers of the MRAM device for efficient reading and writing operation, the conductors only need to be sufficiently near the regions to which the magnetic field is to be applied such that the magnetic field is effective. Examples of other MRAM devices are described in U.S. Pat. No. 6,714,442 to Nahas, U.S. Pat. No. 6,646,948 to Stence et al., and in U.S. Pat. No. 5,734,605 to Zhu et al., the subject matter of each of which is herein incorporated by reference in its entirety.
The MTJ cells in MRAM devices described by Engel et al. include a bit magnetic region, a reference magnetic region, and an electrically insulating material that forms a layer that acts as a tunneling barrier, as well as those portions of the word line and digit line that carry currents that affect the operation of the MRAM device. The bit magnetic region and reference magnetic region may each comprise more than one layer, some of which can have a magnetic moment associated therewith. Some conventional MRAMs have a bit magnetic region that is a single ferromagnetic layer, and other conventional MRAMs have a bit magnetic region that is a multilayered unbalanced synthetic anti-ferromagnetic region. The bit magnetic region and reference magnetic region are positioned adjacent to the tunneling barrier, on opposite sides thereof.
A resistance of the MTJ device is determined by the relative polarities of a bit magnetic moment and a reference magnetic moment. The bit magnetic moment is positioned where the bit magnetic region is adjacent to the tunneling barrier. The reference magnetic moment is positioned where the reference magnetic region is adjacent to the tunneling barrier. The bit magnetic region may be a free ferromagnetic region, meaning that the bit magnetic moment is free to rotate in the presence of an applied magnetic field. The bit magnetic moment has two stable polarities (states) in the absence of any applied magnetic fields along a magnetic axis, known herein as the “easy axis”, determined at the time of deposition of the magnetic material and fabrication of the magnetic regions of the MRAM array. An axis orthogonal to the easy axis is known as the “hard axis.”
MTJ devices also have potential applications as magnetic field sensors. In this instance, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. U.S. Pat. No. 5,650,958 granted to Gallagher et al., the subject matter of which is herein incorporated by reference in its entirety, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect. An example of another MTJ sensor is described in U.S. Pat. No. 6,822,838 to Lin et al., the subject matter of which is herein incorporated by reference in its entirety.
One of the major drawbacks to the use of MTJ in MRAM devices and magnetic sensor devices is that MTJ's generally require a plurality of layers, which increases the cost and complexity of forming such devices. Thus, it would be desirable to achieve the benefits realized by MTJ in a more straightforward and streamlined fashion.
Ferromagnetic mixed valence manganites with a perovskite crystalline structure have received considerable attention because of their interesting magnetic and magnetoresistive properties and in particular the colossal magnetoresistance (CMR) effect. Magnetoresistance is a parameter that describes the percentage change in the resistance of a system in the presence of, and in the absence of, a magnetic field. Thin films of these ferromagnetic manganites generally have the formula R1-xAxMnO3, where R is a rare-earth ion and A represents an alkaline-earth ion. The rare-earth ions are typically lanthanides, although other rare-earth ions are also usable, and the alkaline earth ions are generally selected from calcium, barium, and strontium. In thin films of these materials, the magnetic anisotropy out of the film plane (op) as well as in plane (ip) plays an important role in potential applications. The anisotropy energy in single crystalline epitaxial films of these materials depends not only on doping, stoichiometry, temperature, and film shape, but also on strain and, therefore on the substrate material, film thickness and deposition parameters.
The CMR-effect is an intrinsic effect with a peak at the Curie temperature, TC. The “Curie temperature” is defined as the critical temperature, below which a spontaneous magnetization involving a parallel alignment of spin moments, which is also termed a “ferromagnetic alignment,” occurs. At temperatures above TC, spins will be randomly oriented, and compounds that exhibit a CMR-effect will be typically insulators (semiconductors). At temperatures below TC, they will typically be ferromagnetic metals. Mixed valence (perovskite) manganite compounds have been studied intensively because of the colossal magnetoresistance (CMR) effects which are found at temperatures around the combined paramagnetic-ferromagnetic and insulator-metal (IM) transitions.
The negative MR that occurs in the case of CMR is attributable to the reduction in spin disorientation that occurs. The electrical conductivity that occurs is due to electrons “hopping” between Mn3+-sites and Mn4+-sites via the “double exchange” mechanism, and the “hopping” probability is at its maximum when the magnetic moments of the two Mn-atoms are aligned parallel to one another, as in the ferromagnetic case. In the presence of an applied magnetic field, the probability of “hopping” will increase as the degree of ferromagnetic alignment increases, and resistance will decrease. The effect is thus usually greatest at the Curie temperature.
Another factor that affects CMR materials is electron-lattice coupling. For the Mn3+, with three electrons in the energetically lower spin triplet state and the mobile electron in the energetically higher doublet state, a Jahn-Teller distortion of the oxygen octahedron can lead to splitting of the doublet; for the Mn4+, the energy of the (empty) doublet can be lowered by coupling to a breathing mode of the lattice. Both effects tend to trap the electron in a polaronlike state, which is in competition with band formation due to ferromagnetic correlations. When the temperature is lowered through the ferromagnetic transition at the Curie temperature (TC), the high resistance polaron state breaks up and changes into a lower resistive ferromagnetic metal state. A second important connection between crystal structure and insulator-metal transition lies in the dependence of the Mn—Mn electron transfer on the Mn—O—Mn bond angles, or equivalently on the orientation of the oxygen octahedra with respect to the main crystal axes. This results in a strong dependence of TC on either external pressure or mean A-site ionic radius, for which a universal phase diagram (at constant doping) can be constructed.
Despite progress in understanding the role of double exchange and electron-lattice coupling, a comprehensive understanding of the transport mechanism in manganites is still lacking.
For thin films of the perovskite manganite compounds, maximum magnetoresistive values in films are usually larger than in the equivalent bulk materials, which is generally believed to be due to structural disorder from, for example, nonepitaxial growth or partial strain relaxation. Many times, the film thicknesses are on the order of 100 nm or less, making strain relaxation likely. Very thin films (≈10 nm), however, can be uniformly strained by an underlying substrate, and it is possible to predict their physical properties. These properties may be different from the changes induced by hydrostatic or chemical pressure, since in-plane (epitaxial) strain leads to an out-of-plane strain of different sign. Strain can be used to induce properties outside the bulk phase diagram.
The inventors of the present invention have determined that the properties of such manganite thin films and in particular their giant planar Hall effect and the existence of biaxial magnetic anisotropy as well as of other films exhibiting similar properties make them good candidates for use in magnetic bit cells for MRAM devices and for use as the active area in magnetic sensor devices.